1. Field of the Invention
The present invention relates to optical substrates having a structured surface, particularly to optical substrates for brightness enhancement, and more particularly to brightness enhancement substrates for use in flat panel displays having a planar light source.
2. Description of Related Art
Flat panel display technology is commonly used in television displays, computer displays, and handheld electronics (e.g., cellular phones, personal digital assistants (PDAs), etc.). Liquid crystal display (LCD) is a type of flat panel display, which deploys a liquid crystal (LC) module having an array of pixels to render an image. Referring to FIG. 1, a common backlight apparatus 10 for LCD comprises a reflector 11, a light guide 12, a light source 13, various optical films including lower diffuser sheet 14, two crossed brightness enhancement films 15 (e.g., two sheets having similar surface structures, with the sheets offset by 90 degrees about an axis perpendicular to the plane of the sheets), and an upper diffuser 17.
The brightness enhancement films 15 use micro-structures to direct light along the viewing axes (i.e., normal to the display), which enhances the brightness of the light viewed by the user of the display and which allows the system to use less power to create a desired level of on-axis illumination. Heretofore, brightness enhancement films have a light input surface that is smooth, through which light enters from the backlight module, and a structured light emitting or output surface provided with micro-structures (e.g., prisms, lenticular lenses or pyramids). The micro-structures provided at the light emitting surface changes the angle of the film/air interface for light rays exiting the films and causes light incident obliquely at the light input surface of the films to be redistributed in a direction more normal to the light emitting surface of the brightness enhancement films.
The brightness enhancement films may be in the form of micro-prismatic sheets. The composition of a micro-prismatic sheet (such as 3M™ prismatic brightness enhancement films) generally comprises two layers, including a PET substrate 16 and a structured layer 18 (e.g., an acrylic layer) having micro-prism structures. The function of the structured layer 18 having a light output surface with micro-prism structures is to collect the light toward the viewer after being scattered by the lower diffuser 14. The PET substrate 16 is a relatively stronger layer that supports the relatively weaker micro-prism layer 18.
It has been proposed to apply one multifunctional film to replace two or three optical films mentioned above. The multifunctional film needs to achieve both functions of light enhancement and the diffusion function of upper/lower diffuser. Heretofore, various approaches had been described in the prior art.
FIG. 2 illustrates a prior art “Type-A” multifunctional film 20. Particles are added to the acrylic prism layer 28 supported by the substrate 26. (See, for example, U.S. Patent Publication No. US2007/0121227.)
FIGS. 3 and 4 illustrate prior art “Type B” multifunctional films. For the multifunctional film 30 illustrated in FIG. 3, a separate coating 32 of a resin containing particles/beads is applied to the bottom surface of the substrate 36 that supports the prism layer 38. Similarly, in FIG. 4, a separate coating 42 of particles are applied to the bottom surface of the substrate 46 that supports the structured layer 48. (See, for example, U.S. Pat. Nos. 5,995,288, 6,147,804, 6,333,817, 6,560,023, 6,700,707, 6,825,984, 6,280,063, JP3968155, JP3913870, and JP3860298.) The Type-B multifunctional film is generally made by depositing an UV curable resin onto the PET substrate and then embossing with a master mold (see, e.g., U.S. Pat. No. 5,183,597) to form the structured layer, and using a conventional solvent casting method to form the additional layer resin layer having particles.
FIG. 5 illustrates a prior art “Type C” multifunctional film 50, which has structured surface 58 and diffusing surface 52 integrally formed on top and bottom sides of a substrate sheet 56. The structured surface 58 and diffusing/scattering surface 52 may be produced by extruding or calendaring the sheet 56 between two rolls or belts with different patterns corresponding to the structured surface 58 and diffusing/scattering surface 52. U.S. Pat. No. 6,280,063 discloses hot embossing micro-prisms to form the structured surface 58 and the diffusing/scattering surface 52 on extruded sheet 56.
U.S. Pat. No. 5,598,280 discloses another example of a Type-C multifunctional film 60, in which a light-diffusing surface 62 is free from light diffusing agent particles. The surface 58 having prism structures with projections are integrally formed with the light-diffusing surface 62 at the back surface of the substrate material 66.
U.S. Pat. No. 5,598,280 also discloses an alternate embodiment, a Type-D multifunctional film 70, in which surface micro-prism projections 78 are formed on the top surface of the substrate 76, integrally with forming the light-diffusing surface of a separate coating layer 72 after it has been applied to the back surface of substrate 76.
The multifunctional sheets mentioned above all have their shortcomings. In particular, optical coupling effect (e.g., the presence of Newtonian rings) is the primary problem with the type-A sheets due to their flat, non-structured bottom surface. The type-B and type-D sheets all have to be made with a multiple-pass process, which makes them more susceptible to defect formation, not to mention increasing production costs. For manufacturing type-C sheets, due to the nature of extrusion processes and the single sheet of material configuration, it is more difficult to make different structures on both side of the sheet simultaneous, and the light enhancing and diffusing capabilities at the respective sides of the single layer configuration is generally less than that of multi-layer configuration.
What is needed is optical structure that both enhances brightness and provides effective diffusion, and overcoming the shortcomings of the prior art multifunctional optical sheet.